1. Field of the Invention
The present invention relates to a method and apparatus for continuously granulating thermoplastic resin material with a screw type kneading and extruding machine and a granulating device.
2. Background
In order to granulate a large quantity of thermoplastic resin (not less than 1000 kg/Hr) that has been polymerized by a polymerizing apparatus, in general, one set of screw type kneading and extruding machine and one set of granulating device are used being combined into one unit. In the screw type kneading and extruding machine, a single shaft screw or two shaft screws are rotated in a cylinder, and when necessary, a gear pump is attached to a front end portion of the screw type kneading and extruding machine so as to intensify the extruding function. The granulating device is formed into a substantial disk-shape. On the disk-shaped component of the granulating device, there are formed a large number of nozzles penetrating the disk-shaped component in the thickness direction, and these nozzles are annularly distributed on the disk-shaped component. The granulating device includes: a die attached onto a front end plane of the screw type kneading and extruding machine; and a cutter unit driven and moved along an outer surface of the die (an extruding plane). The granulating device is composed as described above, and there are two types of granulating devices. One is a submerged type granulating device, and the other is an aerial type granulating device.
By the screw type kneading and extruding machine and the granulating device constructed as described above, pellets of thermoplastic resin are made as follows. Powdered thermoplastic resin material is continuously fed from a polymerizing apparatus. Then the thermoplastic resin material is kneaded and melted by the screw type kneading and extruding machine. After that, the thermoplastic resin material is extruded from the nozzles of the die being formed into the shape of pieces of string.
In the granulation of thermoplastic resin, in order to simplify the arrangement of the apparatus and also in order to enhance the efficiency of the operation of the apparatus, one set of polymerizing apparatus is directly connected with one set of screw type kneading and extruding machine. In this case, after the resin material has been polymerized by the polymerizing apparatus, it is directly fed to the screw type kneading and extruding machine and pelletized by the granulating device.
Since the conventional granulating device is constructed as described above, the following problems may be encountered. When the physical property, that is, the viscosity of thermoplastic resin material fed from the polymerizing apparatus is greatly changed, or alternatively when a quantity of thermoplastic resin material fed from the polymerizing apparatus is greatly changed, it is impossible to normally conduct granulation with one type die. When granulation is conducted with one type die, the following problems may be caused. It is impossible to produce spherical pellets, and comma-shaped pellets are produced; plate-shaped pellets are produced; pellets are curled; thread-shaped objects are attached to pellets; and pellets are not separated from each other but they are connected to each other. In this way, defective pellets are produced.
In general, the granulating device including a die and cutter unit is designed so that the processing capacity of the granulating device is equal to the processing capacity of the screw type kneading and extruding machine. That is, when the viscosity of resin material fed from the polymerizing apparatus changes in a wide range, the specification of the granulating device is determined so that the resin material of high viscosity, which requires much energy when it is processed in the screw type kneading and extruding machine, can be normally granulated. In the case of processing resin material of high viscosity, the pressure loss is relatively large when it is fluidized and extruded from the nozzles of the die. Accordingly, in order to reduce the pressure loss, the number of the nozzles is increased, so that the flow velocity is decreased. In the case of processing resin material of low viscosity, the pressure loss is relatively small when it is fluidized and extruded from the nozzles of the die. Accordingly, the resin pressure in the die is low. Therefore, the resin material is extruded from the die as a soft and weak string-shaped resin. For this reason, it is difficult to smoothly cut the resin material with a cutter, and defective pellets are produced. In order to take countermeasure against the above problem, the number of nozzles is reduced, so that the flow velocity of resin in the nozzle can be increased. In accordance with the difference of the die and also in accordance with the difference of the flow velocity of resin material, the number of cutters of the cutter unit or the rotating speed (the moving speed) of the cutters is differently determined. That is, in the case of processing resin material of high viscosity, the number of cutters is reduced and the rotating speed is increased, because resin material extruded from a large number of nozzles at a low flow velocity is cut. In the case of processing resin material of low viscosity, the number of cutters is increased and the rotating speed is decreased, because resin material extruded from a small number of nozzles at a high flow velocity is cut.
Thermoplastic resin is produced by a series of polymerizing apparatus while the polymerizing condition is changed so as to produce resin materials of different viscosity. That is, several types of resin materials, the viscosity ranges of which are different, are successively or alternately produced. In some cases, due to a decrease in the demand, a reduction of output is forced. However, in any cases, the polymerizing apparatus is not stopped even in a short period of time, but it is continuously operated by changing the polymerizing condition in accordance with the change in the production. It is necessary for the screw type kneading and extruding apparatus directly connected with the polymerizing apparatus and also it is necessary for the granulating device attached to the screw type kneading and extruding apparatus, to be operated for conducting granulation normally in accordance with a change in the viscosity of resin material fed from the polymerizing apparatus and a change in the quantity of resin material fed from the polymerizing apparatus. That is, it is necessary for the screw type kneading and extruding apparatus and the granulating device to produce pellets, the shapes of which are not defective. When the resin material feeding condition is changed, the screw type kneading and extruding machine can be adopted to the change by changing the number of revolutions of the screw or the temperature condition of the cylinder. Although the granulating device can be adopted to a narrow range of change by changing the rotating speed of cutters, however, it is impossible for the granulating device to be adopted to a wide range of change. Consequently, it is possible to consider preparing several granulating devices, the applicable viscosity ranges of which are different, and replacing the granulating device in accordance with the condition. However, in order to replace the granulating device, it is necessary to temporarily stop the operation of the screw type kneading and extruding machine. However, in the case of a screw type kneading and extruding machine directly connected with a polymerizing apparatus, the screw type kneading and extruding machine can be stopped only for a short period of time since the polymerizing apparatus is continuously operated. In such a short period of time, it is impossible to replace the granulating device. When the granulating device is replaced if necessary, operation of the polymerizing apparatus is reduced, and resin material polymerized in the process of the reduced operation is discharged. In some cases, the discharged resin material is wasted.